Wave amplifying system



April 9, 1940. J, M, w s-r WAVE AMPLIFYING SYSTEM Filed April 26, 1939/Nl ENTOR J. M. WEST ATTORNEY l PatentedApr; 9, 1940 a a 1 I I "35 willbe regarded herein as the amount of .feed- "l -b li t d rs l su phidebrother temperature UNITED STATES OFFI E -Q f i1 Bell TelephoneLaboratories, Incorporated,

New York, N. Y.', a corporation of New York I Application April 26,1939, Serial ;No.p 270;054

. I 7'C laims. (01. 179 -171) I I v This invention relates towaveamplifying sys a variable impedance element common'to the [L j terns, asfor example, vacuum tube amplifiers. circuit and the p circuit forvarying the amplificae Objects 'of-the invention-rare to controltranstion of the amplifier while maintaining .98 conmission propertiesofsuch systems, for example Stant. I I v 5 gain anddis'tortionintroduced by the systems, to Forexample, in a regulating-amplifieriwith an 52 facilitate application of feedback in the systems,I equalizer i the B C c t compensate Variaandto,reducetsinging'tendencyor maintain proptions o-fcable'losses due totemperaturevariaer margin againstsinging in the systems. tions, thedesired regulationfo'f the amplification In one specifieaspect theinvention is applied of the amplifierisaccomplished withoutmaterial toan amplifier with a forwardly transmitting variation 01" 3 by varying aimpedance common 10- wave amplifying portion and a' feedbackpath to thea circuit and the ecircuitsoas to change from the output tothe input ofthe amplifying it and {3 simultaneously and substantially equallyportion for producing negative or gain-reducing bufiiHVBISe Y- Forin cahe m ififl ybfi feedback around thefamplifying portion, for exathree-stage amplifier with this simultaneous ,u I I ample tostabilizethe gain or reduce themodulacircuit a ndfi C 1 fited by 3 Vaiation introduced by; the amplifier, in accordance .Ible mpedanc CO m tot m circuitand-the, with the'principles' described byII-I. S. Black inhis {3 circuitgvarying' the meal negative feedback on' I articleentitled Stabilized feed-back amplifiers, the third sta e and su st ly qy u i B'eli System Technical Journal, January 1935i, and, verselyvarying the negative feedback around all,"

in his Patent 2,102,671, December 21, 1937. q I of't he stages I I wWith sufiicient feedback, the amplification of n c ordan -W m ature o thinv n n such an amplifier varies substantially asthenegae' a singlevariable elementin a feedback amplifier tive reciprocal of p, thepropagation through the V the am i c fv amplifier While MB feedback pathor 13 circuit, and-is substantially f em nsfixed. Thi$haslmairkedadvantagesvover 5 independent of t, the amplificationof theamplius of ate va elements in t rc fying portion era circuit; for-example, thedecibel 'a e e irc ta a cess tyiSavQidgain of the amplifiervariessubs'tantially as the' ed for calibration of an .element in thelp.circuit. decibel loss in the feedback path. It is usual to a d anelement in e e i u t at v t6 each controlthe-amplificationoftheamplifier for any to m them k; n t moreo-ver,

'39 given. frequency by changing [i -for suchxfrequeny flange! Ofthe mpifi be0mingfln$l7ab16due t However, this change causes Variation inth'efailure (if oneiotf them Q dfeedback factor or 18 gain (i. e., in 5 thecomplex I e, mpe anc iniaac rdan e W th quantity by which a voltage ismultiplied in a e invention'may 60 y t l 'a i single journey around thefeedback loop, which for e p c ya flm -1" fi a t back), and the.variation in the'feedback'may' be. a d pende t resistance, a be u Th ale -I objectionable. For example, it may unduly erem p i l h j d li ytyl nyr duce the singing margin or the amountof ridodu-v u tab e aut atato as for instanc a" lation reduction or. amplifier gain stabilit on:pi o twir r pi t5 a 1j u1a 0r-- in tained. by the feedbackiaction'; orit may Qbj ec-I Other objects and features oftheinvention will I Itionably changethe'amplifieninput 'or output b app rentl r m; th fo lowng'd s riptionand. impedance orvtheIefiect (discussed hereinaften'claims- :I I i which has beenl called them? finitude-eifect, an w 7 III effect relating to departure of the amplification *F acilcuit diagramof a m lifiet f am Q of the amplifier from the value I known type,for-facilitating explanation oi thef4 4 I a 1 I I I invention andof anexample of its application; j V? I I ,f I "Fig. 2 is ablock schematiccircuit diagram of I B an; amplifier embodying a form'of'the invention;.(due to thefinitude of as). and

'50 Objecticnablevariation of the ieedbackcan 'be I Figs. Band 4-areblock schematic diagrams of 56: i avoided by making "equal but inverseor comple modifications of theamplifier of Fig. 2; mentary changesin LL!and. p,- so that s remains Fig; 1 shows a typical three-stageamplifierem- '7 constant. a "j plo ying-series type negative feedback, with itsIn onespecifieaspcct the invention is astagain adjustable by av"arial:1le impedance Z"v in 'bilized feedback amplifier'with meanscomprising the p -circuit I'he amplifier comprises vacuumj 5f networksZr and Z2. source or sending circuit of electromotive force E v tubesV-I, V-2 and V 3 cascaded by coupling It amplifies waves from a andimpedance Zin and transmits the amplified waves to a load circuit orimpedance Zout. The

third stage has a cathode network or impedance ter. Their designationsare taken as their vector values. In Fig. 1

in+ ziut insertion gain- Z v M-i- MW and the insertion gain of theamplifier given by: y

Substituting from (1) and (5 in (,2) gives:

I Z Z i V fl3 3 m3 03 fim-lm] Solving (6) for eg: gives:

Substituting from (7)"in (1)"yields:

Substituting from (10) in (3) gives: I

where S1111, Smz and Sm3 are the transconductances V Z Z of tubes Vl; v2, and v-3.1 smaa(z,, Fig. 2 is like Fig. 1 except: (1) the cathode nett' =,5',, Z S,, Z g or impedance of the third tube has been made L +,g(zm+ ifl into'or replaced by'one composed of impedances ZV+ZKZ+Z2 Em andZKZ in series which have the same con 7 (11) gurations, respectively, asZ 1 and Z 2 or which x have the impedance-frequency characteristics ofysolvmgul) for I Zm and ZK2 the same shapes as those of Z 1 and e SMZSMZZ E' Z 2, respectively,- over the frequency range for 3 Z Z Z which eis to be maintained constant; and (2) Z S "F +Z +ZKZ+Z,; instead ofleaving Zv in circuit and shunting Zxz 1 2 t- Z Z with a variableimpedance for producingVaria- ZTTZE tions in [L equal but inverse tothose produced in e g l v by variation of Zv, Zv is omitted and the two(12) points where the variable elements across Zxz and Substituting from(12) in (8) and rearranging Z 2 would respectively connect to the"junction of terms'gives:

Sm1 1 m2 2 3 Z Z Q V+ K2+ fi2 7,1 1+5 ZS Z SW13 v m (13) 7 1 M 1+ ZVZKZ2+ sz) M In V+ K2+ fl2 2x1 withZKzfand the junction-of Z 1 and Z 2 areconnected through a single variable element or impedance Zv, or in otherwords, Zv is omitted and a variableim'pedance Zv is shunted across ZxzandZ z. 4 When ZK2 is K times as large as Z and Zv'is equal to (1+K) Zvthe new configuration shown in-Fig. 2 will give exactly the sameinsertion gainas that indicated-above for Fig. 1. If ZKI is K times'aslarge as Zpl, Z'Kz is K times as large as Z 2, and SmZm is largecompared to unity, p will be substantially independent of the setting ofZv. Proof of this, and derivation of an expression for the insertiongain in Fig. 2 are given below.

f Referring to Fig; 2 we can set down the following expressionszt Sincethe denominator in (13)v is equal to l-lc and 3 is much greaterthanunity 'we may sim- Forthe reference condition without the amplit Ifier between the source and load we have:

By inspection from (13):

the circuitof Fig. 1.

This is, therefore, a set of conditions th'atmakes ,ufi (as regards bothits phase and its magnitude) independent of Zv.

If Equation (20) is satisfied and in addition the condition 1 gv=(1+K)Zv (22) then I I V in'lT aur 1 xeferenoa fl1+ fl2 ZIV v-F 52which can be shownto be the same as that for Expression (18) issatisfied if the local negative feedback around thelast stage is muchgreater than unity. Thus,,with considerable lo-- cal' negative feedbackand considerable over-all negativerfeedback, an will be substantiallyindependent of adjustment, of Zv, provided Zv is shunted across equalpercentages of the cathode I net impedance ZK1+ZK2 and the ,3circuitiimpedance Z 1+Z,q2, (it being understood that Zm and Z 2 havetheir impedance-frequency character I istics the, .same shapes as thoseof: Z a-and Z 2,

pendent of the setting of Zv.

respectively. The greater the local feedback, the

more nearly will the over-all feedback be inde- The ,8 circuit networkcomposed of impedances 2 1,2 2 and Zv may be made such as to give theamplifier a desired, insertion gain-frequency characteristic, Aprocedure often facilitating this is first to determine the impedancesZn 'Z z and Zv required in the case of Fig. .1 for the desired insertiongain-frequency characteristic,

from the expression given above foninsertion gainof the amplifier inFig. 1, then to determine Zv from the relation Zv= (1+K) Zv,- givenabove as that for making the insertion gains of the amplifiers of Figs.1 and 2 the same. The p circuit network of impedances Z Z 2 and Zv maythus be made to give the amplifier of Fig. 2 an insertion gain which,over the used frequency is flat; or it may be chosen to make theamplifier insertion gain vary in some chosenlinear or nonlinear mannerwith frequency.

The variable impedance .Zv in Fig. Land in Fig. 2 the variable impedanceZv which is equal to (1+K) Zv, may be such that adjustment of thevariable impedance Zv Changes the loss of the e circuit network composedof Z Z52 and Zv equally at all of the usedfrequenciesor. by

' desired different amounts at the difierentIuSed frequencies, (forexample inthe manner .of the adjustable impedance in the variableequalizer network of Fig. 18 of H. W..Bode Patent 2,096,027,

.October 19, 1937, for ,Attenuation equalizer), so

that the adjustment of the impedance "-Z v and consequently theadjustment of Zv Willaccording ly change the insertion gain oftheamplifier equally at all of the used frequencies or by desireddifferent amounts at the difierentusedfrequencies.

trolcircuits.

In this figure, as in the case of Fig. 2, when 3 =islike 'Fig. 2 except:(1) B supply, coupling impedances I and 2, gridleak resistors 3, 4 and5,-and stopping condensers 6, I, 8, 9 and I!) are shown; (2) thesourceor 'sending'circuit j" is shown 'as'input transformer l I connected tocircuit l2 and thedoad is shown as output transformer I3 connected tocircuit [4, either or each of the circuits l2 and 14 being, for example,a

section of coaxial line or cable for broad fre-.

quency band multiplex carrier or television transmission; (3) cathodenetworks or impedances. l5

' and 16 are shown for tubes V-l and V2, re-

spectively, these networks serving, for example,

to provide grid biasing voltages or alternating current feedbackvoltages, or both, for these tubes; and (4) the impedance Zv is shownasatemperature dependent resistance, for exam ple silver sulphide, havingitstcmperature (and consequently'its resistance) variable by means of aheating element 2,0 controlled .by pilot channel or pilot frequencyoperated regulator circuit 2| to vary and e substantially equallybutinversely- This maintains e constant while varying the amplification orthe gain of the amplifier, for example, to compensate for, variations"of cable tem- 1 :perature and cable length. I v I The regulator circuitmay be of any suitable type. For examplait may be of the type disclosedin.E.;I. Green Patent 1,918,390,July 19',

1933, R. W. Chesnut Patent 2,154,062, April '11,

1939, R. R. Blair Patent 2,100,375, November 30, 1937, R. R- BlairPatent 2,179,915, November 14,

1939, for Gain control circuits, or R. R. Blair Patent 2,178,333,October 31, 1939, for Gain con- Zv varies, the local negative feedbackon the third tube varies inthe same sense as the negative feedbackaround. all of "the stages tends to vary due to change. of 8, andthe'amount of the change inthe local feedback is such as tocausethechange inpltoequal the inverse change in [1, so that -,u13 remains]constant as is changed. v z i Q Forsimplifyingthe drawing, inFig. 3, asin the otherfigures, thecircuits for heating the cath-, odes areomitted. -In eachofthe figures they maybe of any usual or suitable type.

.Likewise, for simplifying the drawingin Figs.

-1 and ;2, and alsoin Fig. 4 about to be described,

thecircuitsincluding direct current sources, stopping,condensers,' etc.,for supplying plate currents and providing grid-biasing potentials tocondition theamp-lifier for operation are omitted. In, each of thefigures. they may be of any usual orsuitable type, asfor example, of thegeneral.

.type indicated in Fig. 3.

the amplifier .gain

The invention may be embodied in various forms. For example, Fig. 4shows a modification Jef Fig. 2 in that the impedance Z1 1+Z1 2,composed of' the networks .-Zm and Z112 in the cathode-plate circuit oftUbeLV S is in the oathode-grid circuit of tube V+Zinsteadfof tube V3.

Thuathe network ZICl-FZKB providesa local negative -feedback around thelast two tubes. in the-case of Fig. 2,;with considerable local negative"feedback (which obtains inFig.-4 when ative feedback: the ,cc gainaround the outer "loop will beusubstantially independent of-the ad-'justment' of impedance Zv provided ZKl divided by Zxz'equals Z dividedby 2 2, and the inser- I tion gain of theiamplifier'will be thesameas'in Fig. 1 provided Zv= (1'+K)Z'v."

In Figs-'2 and 4, the variable "impedance Zv ,Sm'JZ2Sm3ZK1 1) ,andconsiderable over-all neg may, if desired, be a temperaturedependentimpedance, such as that of Fig. 3; and in Fig. 4 the adjustment of thisimpedance may, if desired, be automatic, for example, as in Fig.3.

In a feedback amplifier, the p43 finitude effect referred to above isthe factor by which the amplification that would obtain if the feedback;43 were infinite, or more generally if ,ufl

were equal to 1- l,8, must be multiplied in order toobtain the actual orexact value -ufi of the amplification. That is, the amplification withfeedback, AF, is

1 1 1 T-7f t 1 which approaches the exact inverse (or negativereciprocal) of the propagation through the feedback path, asapapproaches infinity. In controlling the gain of the amplifier by varyingZv in accordance with the invention, avoiding change of s (during gainadjustment) avoids varying the factor and thus renders departure of theamplification from independent of the setting of Zv. This isadvantageous, for example, as facilitating equalization or control oftransmission over the circuit in which the amplifier is connected bycontrol of the transmission properties of the feedback path,

for instance as facilitating design of impedances Z91, Z 2 and Zv tocompensate for line or cable attenuation changes, variable withfrequency, due

to changes of line length or of temperature or other weather conditionsto which the line or cable in Fig. 3 is subject. By way of furtherexample, the feature of simultaneously changing a and e substantiallyequally but inversely is advantageous in enabling the over-all gain ofthe amplifier to be changed, by change of the transmission efficiency ofthe feedback path, while fied in said amplifying path, and meansforvarying the transmission efficiency of said feedback path andsimultaneously substantially equally but inversely varying thetransmission efficiency of said amplifying path, said means comprising atwo-terminal temperature responsive impedance common to said paths. 2.An amplifier comprising an amplifying path,

a feedback path around said amplifying. path for producing negativefeedback in said amplifier, a two-terminal impedance in said first path,a two-terminal impedance in said second path, and means for producingsimultaneous inverse changes in propagation through said paths, saidmeans comprising a two-terminal variable-impedance connected across aportion of each of said two first-mentioned impedances.

3. A wave amplifying system comprising an amplifying path, a feedbackpath from the output of said amplifying path to its input for producingnegative feedback in said system, a twoterminal impedance in saidamplifying path, a two-terminal impedance in said feedback path, saidsecond impedance having one of its terminals common with one of theterminals of said first impedance, and means for simultaneouslyproducing approximately equal but inverse changes in propagation throughsaid-paths, said means comprising a two-terminal variable impedanceconnected across substantially equal percentages of said first andsecond impedances.

4. A wave translating system comprising an amplifying path with partlycommon amplifying portions one of which has smaller amplification thananother, a feedback path for producing negative feedback around saidamplifying path, a two terminal impedance so connected in said firstpath as to produce local negative feedback around said one portion, atwo-terminal impedance included in said second path having one of itsterminals common with one of the terminals of said first impedance, andmeans for simultaneously producing approximately equal but inversechanges inpropagation through said paths, said means comprising atwo-terminal variable impedance connected across substantially equalpercentages of said first and second impedances.

5. An amplifier comprising an amplifying path, a feedback path forproducing negative feedback around said amplifying path, cascadedamplifying devices included in said first path, a twoterminal impedanceso connected in said first path, as to produce local negative feedbackaround certain of said devices, a two-terminal impedance included insaid second path having one of its terminals common with one of theterminals of said first impedance, and means for simultaneouslyproducing approximately equal but inverse changes in propagation throughsaid paths, said means comprising a two-terminal variableimpedanceconne'cted across substantially equal percentages of said firstand second impedances.

6. A multistage amplifier comprising a feedback circuit coupling theoutput circuit of the last stage of the amplifier and the input circuitof that stage for producing local negative feedback in that stage, afeedback circuit coupling the output circuit of that stage to the inputcircuit of the first stage for producing over-all negative feedbackaround all of the stages of the amplifier to control the amplifier gain,and a two-terminal temperature responsive impedance connected betweensuch points of said first-mentioned feedback circuit and saidsecond-mentioned feedback circuit that Variation of said temperatureresponsive impedance varies the propagation of said second feedbackcircuit and inversely varies the propagation of said last stage.

7. A multistage amplifier comprising an impedance connected in theoutput circuit of the last stage and having a portion in the inputcircuit of the first stage for producing'pver-all the input circuit ofan intermediate stage for producing local negative feedback around agroup p of stages, and a two-terminal variable impedance electricallyconnecting suchpoints of said portions that variations of said variableimpedance varies the propagation of the' over-all feedback path of theamplifier'and simultaneously.

approximately equally but inversely varies the propagation of said groupof stages.

JULIAN M. WEST.

